A sponge filter is a type of aquarium filter that uses a sponge as its primary filtration medium. It works by drawing water through the sponge, which traps debris and provides a surface for beneficial bacteria to grow. These bacteria help break down harmful substances, making the water cleaner and healthier for fish and other aquatic life. Sponge filters are often used in small aquariums or breeding tanks because they provide gentle water flow and aeration. They are easy to maintain, as the sponge can be rinsed and reused, making them a cost-effective option for both beginners and experienced aquarists.